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Today sucked. You felt tense, stressed, and sulky.

Want tomorrow to be different? Want to be in a better mood the whole day? It’s easy: Work out early, for about 20 minutes, before you start your day.

You probably already know that exercise is energizing. Researchers at the University of Georgia found that previously sedentary adults who started doing 20 minutes of low- to moderate intensity aerobic exercise three times a week for six weeks reported feeling less fatigued and more energized. The time of day doesn’t matter; work out a little, feel a little better. Even five minutes of moderate exercise can create a mood-enhancement effect.

But one study took a different approach, focusing on the longer-term impact of exercise on mood.

Researchers at the University of Vermont found that aerobic training of “moderate intensity,” with an average heart rate of around 112 beats a minute–elevated, sure, but it’s not like they were hammering away–improved participants’ mood for up to twelve hours after exercise.

“Moderate intensity aerobic exercise improves mood immediately and those improvements can last up to 12 hours,” says Dr. Jeremy Sibold. “This goes a long way to show that even moderate aerobic exercise has the potential to mitigate the daily stress that results in your mood being disturbed.”

So there you go: Work out first thing. Feel better. Be smarter. Sure, you could work out after work, but then the happy feelings and extra brainpower will be wasted while you’re asleep.

Remember, you only need to do about 20 minutes of moderate aerobic exercise. For most people, “moderate” means your heart rate should be within 100 to 120 beats per minute (depending on age, fitness level, medical conditions, etc.)

Checking your heart rate is easy. Exercise for about five minutes and then take your pulse. (There are plenty of apps you can use, but any timer is fine.) Or use a Fitbit or FuelBand. Or use the “conversation” test. Say you and a friend are jogging and you’re struggling to make small talk because you’re gasping for air; that means you’re probably working too hard.

The key is to remember that your morning workout doesn’t have to be draining or exhausting. You don’t have to wear yourself out to get a day-long mood and brainpower boost. All you have to do is get up, get moving, and see your quick morning workout not as a chore but as a way to kick-start your day–it’s like coffee, but much more effective and long-lasting.